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Mixed monolayers of phospholipids with a viologen and the electrochemical properties in Langmuir–Blodgett films
Colloids and Surfaces A: Physicochemical and Engineering Aspects 175 (2000) 93 – 98 www.elsevier.nl/locate/colsurfa
Mixed monolayers of phospholipids with a viologen and the electrochemical properties in Langmuir–Blodgett films Dong-Jin Qian, Chikashi Nakamura, Jun Miyake * National Institute for Ad6anced Interdisciplinary Research, AIST, 1 -1 -4 Higashi, Tsukuba, Ibaraki305 -8562, Japan
Abstract Monolayers of phospholipids, 1,2-Didodecanoyl-sn-glycero-3-phosphate (DDGP) and 1,2-dihexadecanoyl-sn-glycero-3-phosphate (DHGP), mixed with a viologen derivative 1,1-Dilauryl-4,4%-bipyridine dibromide have been formed on 0.01 mol l − 1 NaClO4 subphase surface and transferred onto solid substrates by Langmuir-Blodgett method. An ideal mixing of two components could be achieved in molar ratio of 1:1. Electrochemical properties of the viologen in mixed mono- and multilayers have been studied for different parameters: the molar ratios of two components, the number of layers and the alkyl chains of phospholipid. Peak splitting and a large DEp value were observed for the LB films of DDGP/viologen, while not for DHGP/viologen. These electrochemical characters were discussed in terms of the formation of dimers, diffusion of a charged species in thin films as well as the arrangement of viologen. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Phospholipid; Viologen; Monolayers; LB films; Electrochemistry
1. Introduction Phospholipid monolayer as a simple biological model system has become a topic of intense research [1]. Immersion of an active compound into phospholipid monolayer to construct a supramolecular structure provides an increased understanding of a process in membrane. This kind of monolayer assembly can be achieved by the Langmuir – Blodgett (LB) and self-assembled (SA) methods, by which it is possible to immobi-
* Corresponding author. Tel.: +81-298-612558; fax: + 81298-613009. E-mail address: [email protected] (J. Miyake).
lize a wide range of compounds on solid substrates [2]. In the present work, a viologen derivative, 1,1dilauryl-4,4%-bipyridine dibromide, was immersed into phospholipid monolayers by using cospreading technique. The chemical structure of the compounds used is shown in Fig. 1. Viologens are of great electrochemical interest because of three main oxidation states and highly reversibility of their redox reactions, and some applications [3]. Several groups have reported interesting experiments with organized monolayers of some viologens on solid surfaces [4–6]. Likewise, monolayers for mixtures of viologens with other functional molecules have been transferred onto substrates,
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and the molecular arrangement, electron transfer mechanism were discussed [7,8]. We have found that immobilized viologen is an effective electron mediator for electrons transferring from electrode/donor to hydrogenase in LB films [9]. Phospholipids have been used to prepare welldefined mixed monolayers with soluble positively charged molecules to create LB films [7,10]. This is due to charge interaction and hydrophobic interaction between alkyl chains of both compounds. We report in this paper the monolayer behaviors for mixtures of phospholipids and viologen at the air/subphase interface, and the electrochemical properties for viologen immersed in phospholipid monolayers.
2. Experimental details Materials 1,2-Didodecanoyl-sn-glycero-3-phosphate (DDGP) and 1,2-dihexade-canoyl-sn-glycero-3-phosphate (DHGP) were purchased from Sigma Chemical Co. 1,1-Dilauryl-4,4%-bipyridine dibromide was synthesized according to literature method [11]. All reagents were used as received and without further purification. A mixture of chloroform and methanol, ratio 4:1 (v/v), was used as spreading solvent. Ultrapure water from a Millipore Milli-Q purification system (18.3 MV) was used throughout. The mixed monolayers of phospholipid and viologen derivative were prepared by spreading onto 0.01 mol l − 1 NaClO4 subphase surface at 20°C. The surface pressure area (p-A) isotherms were measured by a KSV 5000 minitrough (KSV Instrument, Finland) using a continuous speed for two barriers of 10 cm2 min − 1. The accuracy of surface pressure is 0.01 mN m − 1.
Fig. 1. The compounds used in this work.
The monolayers were transferred onto quartz and tin oxide (ITO) substrates by LB method. The ITO plate was carefully cleaned according to the literature [12]. The dipping speed is 2 mm min − 1. UV-vis spectra were measured by UV1601 UV-vis spectrophotometer (Shimadzu, Japan). Cyclic voltammogram (CV) experiments were performed with a BAS 100B electrochemical analyzer (USA). A Pt wire and Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. ITO substrate, coated with transferred LB films, was used as the working electrode using 0.1 mol l − 1 NaClO4 as electrolyte. An initial potential of − 0.20 V was applied for 2 s, and subsequently several cyclic scans to a final potential of − 1.20 V were obtained. All electrochemical measurements were made under Ar atmosphere at room temperature.
3. Results and discussion
3.1. Beha6iors of the mixed monolayers of phospholipids and 6iologen Since viologen could partly dissolve into pure water, solutions of mixtures of phospholipid and viologen were spread onto the surface of a subphase containing 0.01 mol l − 1 NaClO4 which can largely reduce viologen solubility, as many viologens with one alkyl chain [4,13]. Fig. 2 shows the p-A isotherms for the mixed monolayers of phospholipids and viologen in the molar ratios 1:1 to 9:1 in comparison with those of pure phospholipids and viologen. The curves for phospholipid monolayers were similar to those reported in literature [7]. An expansion with respect to the reference phospholipid isotherm for mixed monolayers was measured, which is clear evidence for the presence of viologen at the interface. The limited area per molecule of DHGP in mixed monolayers with viologen at p= 10 mN m − 1 was 90, 106 and 183 A, 2; and of DDGP was 168, 186 and 228 A, 2 corresponding to the molar ratios 9:1, 3:1 and 1:1 (Fig. 1b, c and d). Considering the molecular area of DHGP, DDGP and viologen at this surface pressure was 48, 75 and
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137 A, 2, respectively, it can be concluded that the average molecular area is almost equal to that from ideal mixing for the monolayers of mixtures with molar ratio 1:1, while for the others it becomes larger than that in ideal mixing. Thus, we can suggest that lipids and viologen are mixed ideally in molar ratio 1:1, resulting in a homogeneously and closely packed monolayer (Scheme 1). When molar fractions of lipids increased, the inhomogeneous charge separation in mixed monolayers may lead to a loosely packed arrangement of molecules. This result is different from those obtained from the mixtures of lipid with viologen having one alkyl chain, where the alkyl chain of viologen is suggested to be retained between lipid molecules without the contribution of the bipyridyl group [7]. Even if the surface pressure is above the collapsed one of pure viologen monolayer, the monolayers of mixtures still have a rather large surface area. So we can conclude that bis-alkylated viologen molecules are immersed into phospholipid monolayers, and the surface coverage is the sum of two components.
3.2. Langmuir–Blodgett films Fig. 2. p-A isotherms for the monolayers of DHGP (A) or DDGP (B), viologen and their mixtures on 0.01 mol l − 1 NaClO4 subphase surface at 20°C. (a) pure DHGP or DDGP; (b) phospholipid: viologen= 9:1; (c) phospholipid: viologen= 3:1; (d) phospholipid: viologen= 1:1 and (e) pure viologen.
Scheme 1. Organization of the mixed phospholipid and viologen monolayers in different molar ratios at the air-water interface. (A) 1:1; (B) above 1:1.
The mixed monolayers were transferred onto hydrophilic ITO and quartz substrates at p=10 mN m − 1 by LB method. From the transfer ratio, it can be found that only when the substrate was taken out from the air-water interface the monolayer can be deposited, indicating a Z-type deposition. Several papers have described the deposition of phospholipids and concluded that a lipid bilayer on solid substrate is not stable [14,15]. This problem still remained in our investigation, though a counter ion compound viologen was introduced into the matrix of lipid monolayer. By measurement of UV-vis spectra of viologen LB films, the p-p* intramolecular transition characters can be described qualitatively. Fig. 3 shows the absorption spectra of LB films of DHGP mixed with viologen (1:1) and pure viologen in comparison with that in ethanol solution. A band at ca. 264 nm in viologen methanol solution is assigned to the p-p* intramolecular transitions at the viologen unit [6]. This band has a rather large red-shift (ca. 14 nm) in pure viologen LB film; while almost no shift was measured for the LB
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films of DHGP mixed with viologen. The red- or blue- shift for a p-p* transition is often observed in the ordered structure, and has been ascribed to the formation of J- or H-aggregate due to the intermolecular interaction [16]. No shift for the p-p* transition of viologen in mixed monolayers indicates weak interaction among viologen molecules, and strongly supports our suggestion on the molecular arrangement in monolayers shown in Scheme 1.
3.3. Electrochemical properties Fig. 3. Absorption spectra for viologen in LB films and methanol solution.
Fig. 4. Cyclic voltammograms of viologen immersed in DHGP (A) and DDGP (B) LB films on ITO electrode at various scan rates.
The ITO electrode, coated with one or several layers of mixed LB films of phospholipid and viologen in the molar ratios of 1:1, 3:1 and 9:1, was immersed in 0.1 mol l − 1 NaClO4 solution and used as working electrode in a conventional electrochemical cell. As a contrast, we also measured the electrochemical properties of a pure viologen monolayer at the same conditions. The CV curves for all cycles of a monolayer are almost the same except for the first cycle, which is a little larger than others. The data discussed below was taken from the 10th cycle. Fig. 4 shows the CV curves for one layer of mixed LB films of phospholipids and viologen (molar ratio= 1:1, as an example) deposited under 10 mN m − 1 at various scan rates. Details on redox potentials are summarized in Table 1. Two pairs of redox peaks for the curve of DHGP/viologen mixed monolayer (1:1) with scan rate 100 mV s − 1 were observed at − 488 mV and −901 mV due to the two, reversible one-electron reductions of viologen derivative, i.e. V2 + V+ and V+ V0 [3]. For pure viologen monolayer, the first reduced peak was split and appeared at − 431 and − 495 mV. The splitting was also observed in the DDGP/viologen mixed monolayer; while the second peak was only a shoulder one. Several papers have reported this splitting in the LB films of viologen derivatives, and have attributed it to some factors, such as the existence of two different conformations of the viologens, interactions between parent and the reduced species, and formation of radical dimers [4–6]. We observed this splitting for DDGP/viologen monolayers, while not for DHGP/viologen monolayer.
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Table 1 The first redox potentials for the mixed LB films of phospholipid and viologen at scan rate 100 mV s−1 Monolayer composition
Number of layers
Epc
Epa
DEp
Viologen
1
−431, −495
−439, −331
DHGP: viologen= 1:1
1 3
−488 −496
−487 −492
1 4
DHGP: viologen= 3:1
1 3
−500 −527
−499 −523
1 4
DDGP: viologen= 1:1
1 3
−419 −425, −480
−346 −472, −352
73
DDGP: viologen= 3:1
1 3
−515 −521
−495 −499
21 22
This may be attributed to that DHGP molecules more effectively separate the viologens, as a result, the interactions among viologen molecules are largely weakened. Furthermore, the relative current intensity of two splitting peaks was closely related to the scan rate, especially for the threelayer films (data not shown). One explanation is the existence of many different potentials in the reduced reaction, due to several kinds of attractive and repulsive interactions among parent and/ or reduced species [6], and the formation of reduced species is closely related to scan rate. According to an equation derived for an adsorbed redox species in a thin-layer cell, ic = n 2CrF 26/(4RT), where n is the number of electrons, Cr is the surface concentration, F is the Faraday constant and 6 is the potential scan rate [17], the current ic should be proportional to the scan rate 6. In the present work, we observed the cathodic current increases linearly with the potential scan rate for one-layer phospholipid/viologen LB film in molar ratio 1:1; while not for a threelayer one when the scan rate is up to 400 mV s − 1, as shown in Fig. 5. The dependence of cathodic peak currents upon the sum of a transferred viologen LB film has been investigated by a quartz crystal microbalance method [18], and found that although the peak currents increased with increasing layer numbers, the absolute peak currents expected from surface coverage of viologen were suppressed. Our observation of a deviation of the linear relationship between ic and 6 is agreement
with the current suppression in a multilayer, and can be ascribed to the slowdown of the electron transfer across the viologen layer and/or the slowdown of diffusion of counter ion within the film. Since a hydrocarbon barrier hinders the solvation of the viologen and the access of the supporting electrolyte to viologen, not all viologens in LB films are found to be electroactive [18]. It is reasonable for the consideration that this hindrance would be enhanced with the film thickness and the scan rate, so the current ic measured at high scan rate is smaller than that expected from the equation. Generally, there is a rather large difference between cathodic and anodic peak potentials
Fig. 5. Dependence of redox peak current on the potential scan rate for mixed LB films of DHGP and viologen.
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(DEp) for a viologen derivative in LB films [6]. However, when the viologen was mixed with another amphiphilic compound or immersed in a mixed self-assembled monolayer, the DEp value could be reduced [7,19]. Ferna´ndez et al. measured the CV properties for the LB films of a lipid mixed with viologen containing one alkyl chain, and found that DEp could be reduced to 30 mV [7]. In the present work, a rather small DEp value was measured as listed in Table 1. As it can be seen that this peak separation is only about 1 4 mV for the DHGP/viologen LB films, even smaller than that from derivatized sulfur viologens in an octadecyl mercaptan matrix on gold surface [19], where the viologen was suggested to be well separated by octadecyl chains. The redox current intensity for viologen immersed in DHGP monolayers is smaller than that in DDGP monolayers (Fig. 4), though there is a higher surface concentration of viologen in the former case according to the p-A isotherms. The long hydrocarbon chains of DHGP/viologen monolayer form a thicker barrier, which makes it more difficult for the access of the supporting electrolyte to viologen. Besides, a smaller occupied molecular area per DHGP/viologen in contrast to DDGP/viologen indicates a more closely packed arrangement in the former case, which hinders the diffusion of counter ion within the film.
4. Conclusion Redox active compound viologen could be immersed into the matrix of phospholipid monolayers based on the charge and hydrophobic interactions. Viologen is suggested to be dispersed into phospholipid monolayers, and an ideal mixing can be achieved as the molar ratio is 1:1. The monolayer can be transferred onto solid substrates by LB method with Z-type deposition. The electrochemical study of the LB films reveals a great dependence of the redox process on monolayer compositions, i.e. molar ratios of two components, alkyl chains and number of layers. The viologen-immersed phospholipid monolayer may .
be an ideal model for the investigation of chemically modified electrode reaction in monolayers.
Acknowledgements This work was supported by Bionic Design Project of NAIR, R&D Project on Protein Assemblies of AIST/MITI, Japan. D.Q. acknowledges additional support from JISTEC, Japan.
References [1] S. Heyse, T. Stora, E. Schmid, J.H. Lakey, H. Vogel, Biochim. Biophys. Acta 85507 (1998) 319. [2] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, New York, 1991. [3] C.-L. Bird, A.T. Kuhn, Chem. Soc. Rev. 10 (1981) 49. [4] C.-W. Lee, A.J. Bard, J. Electroanal. Chem. 239 (1988) 441. [5] S. Ye, J.-H. Kim, R.A. Uphaus, T.M. Cotton, T. Lu, S. Dong, Thin Solid Films 210/211 (1992) 822. [6] P. Cea, C. Lafuente, J.S. Urieta, M.C. Lo´pez, F.M. Royo, Langmuir 14 (1998) 7306. [7] A.J. Ferna´ndez, M.T. Martı´n, J.J. Ruiz, E. Mun˜oz, L. Camacho, J. Phys. Chem. B 102 (1998) 6799. [8] H. Yonemura, K. Ohishi, T. Matsuo, Chem. Lett. (1996) 661. [9] D.-J. Qian, C. Nakamura, K. Noda, N.A. Zorin, J. Miyake, in press. [10] I. Priteo, M.T. Martı´n, D. Mo¨bius, L. Camacho, J. Phys. Chem. B 102 (1998) 2523. [11] X.R. Xiao, C.B. Wang, T.T. Tien, J. Mol. Catal. 23 (1984) 9. [12] R.N. Dominey, T.J. Lewis, M.S. Wrighton, J. Phys. Chem. 87 (1983) 5345. [13] M. Shimomura, K. Kasuga, T. Tsukada, Thin Solid Films 210/211 (1992) 375. [14] H.F. Knapp, W. Wiegra¨be, M. Heim, R. Eschrich, R. Guckenberger, Biophys. J. 69 (1995) 708. [15] S.W. Hui, R. Viswanathan, J.A. Zasadzinski, J.N. Israelachvili, Biophys. J. 68 (1995) 171. [16] H. Tachibana, F. Sato, S. Terrettaz, R. Azumi, T. Nakamura, H. Sakai, M. Abe, M. Matsumoto, Thin Solid Films 327-329 (1998) 813. [17] S. Dong, G. Che, Y. Xie, Chemically Modified Electrodes, Kexue Chuban She, Beijing, 1995, p. 52. [18] N. Oyama, S. Ikeda, O. Hatozaki, M. Shimomura, K. Mishima, S. Nakamura, Bull. Chem. Soc. Jpn. 66 (1993) 1091. [19] J.-H. Kim, K.A. Bunding Lee, R.A. Uphaus, T.M. Cotton, Thin Solid Films 210/211 (1992) 825.
Mixed monolayers of phospholipids with a viologen and the electrochemical properties in Langmuir–Blodgett films Dong-Jin Qian, Chikashi Nakamura, Jun Miyake * National Institute for Ad6anced Interdisciplinary Research, AIST, 1 -1 -4 Higashi, Tsukuba, Ibaraki305 -8562, Japan
Abstract Monolayers of phospholipids, 1,2-Didodecanoyl-sn-glycero-3-phosphate (DDGP) and 1,2-dihexadecanoyl-sn-glycero-3-phosphate (DHGP), mixed with a viologen derivative 1,1-Dilauryl-4,4%-bipyridine dibromide have been formed on 0.01 mol l − 1 NaClO4 subphase surface and transferred onto solid substrates by Langmuir-Blodgett method. An ideal mixing of two components could be achieved in molar ratio of 1:1. Electrochemical properties of the viologen in mixed mono- and multilayers have been studied for different parameters: the molar ratios of two components, the number of layers and the alkyl chains of phospholipid. Peak splitting and a large DEp value were observed for the LB films of DDGP/viologen, while not for DHGP/viologen. These electrochemical characters were discussed in terms of the formation of dimers, diffusion of a charged species in thin films as well as the arrangement of viologen. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Phospholipid; Viologen; Monolayers; LB films; Electrochemistry
1. Introduction Phospholipid monolayer as a simple biological model system has become a topic of intense research [1]. Immersion of an active compound into phospholipid monolayer to construct a supramolecular structure provides an increased understanding of a process in membrane. This kind of monolayer assembly can be achieved by the Langmuir – Blodgett (LB) and self-assembled (SA) methods, by which it is possible to immobi-
* Corresponding author. Tel.: +81-298-612558; fax: + 81298-613009. E-mail address: [email protected] (J. Miyake).
lize a wide range of compounds on solid substrates [2]. In the present work, a viologen derivative, 1,1dilauryl-4,4%-bipyridine dibromide, was immersed into phospholipid monolayers by using cospreading technique. The chemical structure of the compounds used is shown in Fig. 1. Viologens are of great electrochemical interest because of three main oxidation states and highly reversibility of their redox reactions, and some applications [3]. Several groups have reported interesting experiments with organized monolayers of some viologens on solid surfaces [4–6]. Likewise, monolayers for mixtures of viologens with other functional molecules have been transferred onto substrates,
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and the molecular arrangement, electron transfer mechanism were discussed [7,8]. We have found that immobilized viologen is an effective electron mediator for electrons transferring from electrode/donor to hydrogenase in LB films [9]. Phospholipids have been used to prepare welldefined mixed monolayers with soluble positively charged molecules to create LB films [7,10]. This is due to charge interaction and hydrophobic interaction between alkyl chains of both compounds. We report in this paper the monolayer behaviors for mixtures of phospholipids and viologen at the air/subphase interface, and the electrochemical properties for viologen immersed in phospholipid monolayers.
2. Experimental details Materials 1,2-Didodecanoyl-sn-glycero-3-phosphate (DDGP) and 1,2-dihexade-canoyl-sn-glycero-3-phosphate (DHGP) were purchased from Sigma Chemical Co. 1,1-Dilauryl-4,4%-bipyridine dibromide was synthesized according to literature method [11]. All reagents were used as received and without further purification. A mixture of chloroform and methanol, ratio 4:1 (v/v), was used as spreading solvent. Ultrapure water from a Millipore Milli-Q purification system (18.3 MV) was used throughout. The mixed monolayers of phospholipid and viologen derivative were prepared by spreading onto 0.01 mol l − 1 NaClO4 subphase surface at 20°C. The surface pressure area (p-A) isotherms were measured by a KSV 5000 minitrough (KSV Instrument, Finland) using a continuous speed for two barriers of 10 cm2 min − 1. The accuracy of surface pressure is 0.01 mN m − 1.
Fig. 1. The compounds used in this work.
The monolayers were transferred onto quartz and tin oxide (ITO) substrates by LB method. The ITO plate was carefully cleaned according to the literature [12]. The dipping speed is 2 mm min − 1. UV-vis spectra were measured by UV1601 UV-vis spectrophotometer (Shimadzu, Japan). Cyclic voltammogram (CV) experiments were performed with a BAS 100B electrochemical analyzer (USA). A Pt wire and Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. ITO substrate, coated with transferred LB films, was used as the working electrode using 0.1 mol l − 1 NaClO4 as electrolyte. An initial potential of − 0.20 V was applied for 2 s, and subsequently several cyclic scans to a final potential of − 1.20 V were obtained. All electrochemical measurements were made under Ar atmosphere at room temperature.
3. Results and discussion
3.1. Beha6iors of the mixed monolayers of phospholipids and 6iologen Since viologen could partly dissolve into pure water, solutions of mixtures of phospholipid and viologen were spread onto the surface of a subphase containing 0.01 mol l − 1 NaClO4 which can largely reduce viologen solubility, as many viologens with one alkyl chain [4,13]. Fig. 2 shows the p-A isotherms for the mixed monolayers of phospholipids and viologen in the molar ratios 1:1 to 9:1 in comparison with those of pure phospholipids and viologen. The curves for phospholipid monolayers were similar to those reported in literature [7]. An expansion with respect to the reference phospholipid isotherm for mixed monolayers was measured, which is clear evidence for the presence of viologen at the interface. The limited area per molecule of DHGP in mixed monolayers with viologen at p= 10 mN m − 1 was 90, 106 and 183 A, 2; and of DDGP was 168, 186 and 228 A, 2 corresponding to the molar ratios 9:1, 3:1 and 1:1 (Fig. 1b, c and d). Considering the molecular area of DHGP, DDGP and viologen at this surface pressure was 48, 75 and
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137 A, 2, respectively, it can be concluded that the average molecular area is almost equal to that from ideal mixing for the monolayers of mixtures with molar ratio 1:1, while for the others it becomes larger than that in ideal mixing. Thus, we can suggest that lipids and viologen are mixed ideally in molar ratio 1:1, resulting in a homogeneously and closely packed monolayer (Scheme 1). When molar fractions of lipids increased, the inhomogeneous charge separation in mixed monolayers may lead to a loosely packed arrangement of molecules. This result is different from those obtained from the mixtures of lipid with viologen having one alkyl chain, where the alkyl chain of viologen is suggested to be retained between lipid molecules without the contribution of the bipyridyl group [7]. Even if the surface pressure is above the collapsed one of pure viologen monolayer, the monolayers of mixtures still have a rather large surface area. So we can conclude that bis-alkylated viologen molecules are immersed into phospholipid monolayers, and the surface coverage is the sum of two components.
3.2. Langmuir–Blodgett films Fig. 2. p-A isotherms for the monolayers of DHGP (A) or DDGP (B), viologen and their mixtures on 0.01 mol l − 1 NaClO4 subphase surface at 20°C. (a) pure DHGP or DDGP; (b) phospholipid: viologen= 9:1; (c) phospholipid: viologen= 3:1; (d) phospholipid: viologen= 1:1 and (e) pure viologen.
Scheme 1. Organization of the mixed phospholipid and viologen monolayers in different molar ratios at the air-water interface. (A) 1:1; (B) above 1:1.
The mixed monolayers were transferred onto hydrophilic ITO and quartz substrates at p=10 mN m − 1 by LB method. From the transfer ratio, it can be found that only when the substrate was taken out from the air-water interface the monolayer can be deposited, indicating a Z-type deposition. Several papers have described the deposition of phospholipids and concluded that a lipid bilayer on solid substrate is not stable [14,15]. This problem still remained in our investigation, though a counter ion compound viologen was introduced into the matrix of lipid monolayer. By measurement of UV-vis spectra of viologen LB films, the p-p* intramolecular transition characters can be described qualitatively. Fig. 3 shows the absorption spectra of LB films of DHGP mixed with viologen (1:1) and pure viologen in comparison with that in ethanol solution. A band at ca. 264 nm in viologen methanol solution is assigned to the p-p* intramolecular transitions at the viologen unit [6]. This band has a rather large red-shift (ca. 14 nm) in pure viologen LB film; while almost no shift was measured for the LB
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films of DHGP mixed with viologen. The red- or blue- shift for a p-p* transition is often observed in the ordered structure, and has been ascribed to the formation of J- or H-aggregate due to the intermolecular interaction [16]. No shift for the p-p* transition of viologen in mixed monolayers indicates weak interaction among viologen molecules, and strongly supports our suggestion on the molecular arrangement in monolayers shown in Scheme 1.
3.3. Electrochemical properties Fig. 3. Absorption spectra for viologen in LB films and methanol solution.
Fig. 4. Cyclic voltammograms of viologen immersed in DHGP (A) and DDGP (B) LB films on ITO electrode at various scan rates.
The ITO electrode, coated with one or several layers of mixed LB films of phospholipid and viologen in the molar ratios of 1:1, 3:1 and 9:1, was immersed in 0.1 mol l − 1 NaClO4 solution and used as working electrode in a conventional electrochemical cell. As a contrast, we also measured the electrochemical properties of a pure viologen monolayer at the same conditions. The CV curves for all cycles of a monolayer are almost the same except for the first cycle, which is a little larger than others. The data discussed below was taken from the 10th cycle. Fig. 4 shows the CV curves for one layer of mixed LB films of phospholipids and viologen (molar ratio= 1:1, as an example) deposited under 10 mN m − 1 at various scan rates. Details on redox potentials are summarized in Table 1. Two pairs of redox peaks for the curve of DHGP/viologen mixed monolayer (1:1) with scan rate 100 mV s − 1 were observed at − 488 mV and −901 mV due to the two, reversible one-electron reductions of viologen derivative, i.e. V2 + V+ and V+ V0 [3]. For pure viologen monolayer, the first reduced peak was split and appeared at − 431 and − 495 mV. The splitting was also observed in the DDGP/viologen mixed monolayer; while the second peak was only a shoulder one. Several papers have reported this splitting in the LB films of viologen derivatives, and have attributed it to some factors, such as the existence of two different conformations of the viologens, interactions between parent and the reduced species, and formation of radical dimers [4–6]. We observed this splitting for DDGP/viologen monolayers, while not for DHGP/viologen monolayer.
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Table 1 The first redox potentials for the mixed LB films of phospholipid and viologen at scan rate 100 mV s−1 Monolayer composition
Number of layers
Epc
Epa
DEp
Viologen
1
−431, −495
−439, −331
DHGP: viologen= 1:1
1 3
−488 −496
−487 −492
1 4
DHGP: viologen= 3:1
1 3
−500 −527
−499 −523
1 4
DDGP: viologen= 1:1
1 3
−419 −425, −480
−346 −472, −352
73
DDGP: viologen= 3:1
1 3
−515 −521
−495 −499
21 22
This may be attributed to that DHGP molecules more effectively separate the viologens, as a result, the interactions among viologen molecules are largely weakened. Furthermore, the relative current intensity of two splitting peaks was closely related to the scan rate, especially for the threelayer films (data not shown). One explanation is the existence of many different potentials in the reduced reaction, due to several kinds of attractive and repulsive interactions among parent and/ or reduced species [6], and the formation of reduced species is closely related to scan rate. According to an equation derived for an adsorbed redox species in a thin-layer cell, ic = n 2CrF 26/(4RT), where n is the number of electrons, Cr is the surface concentration, F is the Faraday constant and 6 is the potential scan rate [17], the current ic should be proportional to the scan rate 6. In the present work, we observed the cathodic current increases linearly with the potential scan rate for one-layer phospholipid/viologen LB film in molar ratio 1:1; while not for a threelayer one when the scan rate is up to 400 mV s − 1, as shown in Fig. 5. The dependence of cathodic peak currents upon the sum of a transferred viologen LB film has been investigated by a quartz crystal microbalance method [18], and found that although the peak currents increased with increasing layer numbers, the absolute peak currents expected from surface coverage of viologen were suppressed. Our observation of a deviation of the linear relationship between ic and 6 is agreement
with the current suppression in a multilayer, and can be ascribed to the slowdown of the electron transfer across the viologen layer and/or the slowdown of diffusion of counter ion within the film. Since a hydrocarbon barrier hinders the solvation of the viologen and the access of the supporting electrolyte to viologen, not all viologens in LB films are found to be electroactive [18]. It is reasonable for the consideration that this hindrance would be enhanced with the film thickness and the scan rate, so the current ic measured at high scan rate is smaller than that expected from the equation. Generally, there is a rather large difference between cathodic and anodic peak potentials
Fig. 5. Dependence of redox peak current on the potential scan rate for mixed LB films of DHGP and viologen.
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D.-J. Qian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 175 (2000) 93–98
(DEp) for a viologen derivative in LB films [6]. However, when the viologen was mixed with another amphiphilic compound or immersed in a mixed self-assembled monolayer, the DEp value could be reduced [7,19]. Ferna´ndez et al. measured the CV properties for the LB films of a lipid mixed with viologen containing one alkyl chain, and found that DEp could be reduced to 30 mV [7]. In the present work, a rather small DEp value was measured as listed in Table 1. As it can be seen that this peak separation is only about 1 4 mV for the DHGP/viologen LB films, even smaller than that from derivatized sulfur viologens in an octadecyl mercaptan matrix on gold surface [19], where the viologen was suggested to be well separated by octadecyl chains. The redox current intensity for viologen immersed in DHGP monolayers is smaller than that in DDGP monolayers (Fig. 4), though there is a higher surface concentration of viologen in the former case according to the p-A isotherms. The long hydrocarbon chains of DHGP/viologen monolayer form a thicker barrier, which makes it more difficult for the access of the supporting electrolyte to viologen. Besides, a smaller occupied molecular area per DHGP/viologen in contrast to DDGP/viologen indicates a more closely packed arrangement in the former case, which hinders the diffusion of counter ion within the film.
4. Conclusion Redox active compound viologen could be immersed into the matrix of phospholipid monolayers based on the charge and hydrophobic interactions. Viologen is suggested to be dispersed into phospholipid monolayers, and an ideal mixing can be achieved as the molar ratio is 1:1. The monolayer can be transferred onto solid substrates by LB method with Z-type deposition. The electrochemical study of the LB films reveals a great dependence of the redox process on monolayer compositions, i.e. molar ratios of two components, alkyl chains and number of layers. The viologen-immersed phospholipid monolayer may .
be an ideal model for the investigation of chemically modified electrode reaction in monolayers.
Acknowledgements This work was supported by Bionic Design Project of NAIR, R&D Project on Protein Assemblies of AIST/MITI, Japan. D.Q. acknowledges additional support from JISTEC, Japan.
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